1. Field of the Invention
This invention relates generally to phase-locked looped systems and more particularly, it relates to an improved digital phase lock detector having a micro-controller based processor unit for determining the period of an incoming signal for use in measuring rapidly and accurately the relative speed or velocity of a target vehicle according to the Doppler-radar principle.
2. Description of the Prior Art
As is generally known in navigational systems, radar (an acronym for Radio Detecting and Ranging) equipment has been used for many years. These conventional radar equipment make use of the known propagation rate of microwave energy to ascertain the distance from the radar unit to a target (i.e., the vehicle under observation). The distance is based upon the fact that it is proportional to the time for the microwave energy to propagate from a source (radar unit) to the target and for the resultant reflected energy to return.
In recent years, there have also been developed radar systems which are capable of determining the speed of the target even when either the target vehicle and/or radar unit is in motion. These prior art radar systems measure the relative velocity of the target and relies upon the frequency shift called "Doppler shift" which the microwave energy undergoes when it is reflected from the target having a relative velocity with respect to the source. The relative velocity is the approaching or receding speed of the target. For example, such applications include Doppler-type speed meters commonly employed by law enforcement personnel to monitor the relative velocity of motor vehicles in order to enforce highway speed limits.
In particular, an ultra-high frequency radar signal is radiated in these prior art radar systems toward the target vehicle under observation. When the transmitted radar signal comes into contact with the target vehicle, a portion of the transmitted Doppler wave is reflected back to the radar unit. Then, the reflected wave is received and mixed with a sample portion of the transmitted wave to measure the difference in frequency between the reflected signal and the transmitted radar signal. This frequency differential is caused by the relative motion of the target vehicle with respect to the radar unit and is proportional to the ground speed of the target vehicle. The amount of this frequency shift may be quite significant. For instance, in an X-band transmitter having a source frequency of 10.525 GHz the amount of shift is on the order of 31 Hz for each mile per hour of relative velocity.
Further, it is also generally known that the basic problem encountered in the detection of the Doppler return signal is due to its inaccuracy because of large noise levels being injected therein from energy reflected by extraneous moving objects and white noise. In an attempt to solve this problem, there have been employed in the prior art radar equipment phase-locked looped techniques which are useful where the accurate detection and measurement of the variable Doppler frequencies returned from or initiated by moving targets are required. The phase-locked loop (PLL) is essentially an oscillator whose frequency is locked onto one single frequency component of an incoming signal. A basic PLL system typically incorporates a voltage-controlled oscillator (VCO), a phase detector, and a low pass filter. The phase detector is utilized to compare the reference frequency of the VCO and the frequency of the incoming signal and provides an error voltage. This error voltage is usually delivered to the low pass filter and then applied to the input of the VCO. The frequency of the VCO is responsive to this error voltage so as to lock the output frequency and phase of the VCO onto the incoming signal.
These prior art Doppler radar systems utilizing the PLL techniques require a finite amount of time before the loop of the system settles on the reference frequency of the incoming signal. The frequency of the voltage-controlled oscillator must be either increased or decreased in order to match the Doppler incoming signal. The locked condition is achieved only when the output of the VCO is in phase with the incoming signal. Accordingly, they suffer from the disadvantage that the speed measurement calculations tend to be relatively long. Further, since these prior art systems were implemented using all analog and discrete digital circuitry to perform a complex function, they lack versatility and flexibility so as to adapt to change in the radar system or application.
A prior art search directed to the subject matter of this application in the U.S. Patent and Trademark Office revealed the following U.S. Letters Patent:
______________________________________ 3,271,767 4,172,256 4,600,889 3,713,149 4,276,548 4,626,857 3,715,751 4,321,602 4,672,330 3,728,723 4,335,383 4,724,437 3,885,238 4,359,734 4,788,547 4,021,804 4,429,309 5,034,748 4,072,947 4,510,463 Re.31,851 4,159,475 4,599,618 ______________________________________
In U.S. Pat. No. 4,335,383 to Berry issued on Jun. 15, 1982, there is disclosed a moving doppler radar unit which separates the incoming Doppler signal into its respective frequency components by means of frequency translation techniques and fixed frequency filters. The received Doppler signal is frequency translated by a modulator 50 (FIG. 2) to a preselected reference frequency. The frequency translated platform speed and the target speed frequency components are then separated by means of a narrow band filter 54 and a bandpass filter 56. The separated frequency components are frequency translated downward by demodulators 74 and 76 so as to obtain the speed signals representative of the ground speed of the target and platform vehicles, respectively.
In U.S. Pat. No. 4,724,437 to Jones et al. issued on Feb. 9, 1988, there is disclosed a signal acquisition circuit for a missile guidance system which includes an adaptive phase lock loop APLL. The phase lock loop is comprised of a phase sensitive detector PSD1, a variable gain circuit VG1, a loop integrator LI1, and a voltage controlled variable frequency oscillator VCO. The APLL is responsive to incoming signals received from a target and will lock on to the frequency of the received signal. The bandwidth of the APLL is altered in dependence on the signal being received in order to distinguish between a valid target and noise.
In U.S. Pat. No. 4,159,475 to Andre et al. issued on Jun. 26, 1979, there is shown a phase lock system for use in a radar system which includes a variable controlled oscillator 166 (FIG. 2) which is locked in frequency and phase to a reference local oscillator 116 in a closed loop configuration.
In U.S. Pat. No. 4,172,256 to Pacozzi issued on Oct. 23, 1979, there is shown a circuit for speed measurement of vehicles according to the Doppler-radar principle wherein a Doppler-useful signal S.sub.N is evaluated in a computer RN to which such signal is infed by means of a threshold value switch SW. A single-sideband suppressed-carrier modulator SSB (FIG. 2) has its input side connected to receive the Doppler-useful signal and also a high-frequency signal S.sub.HF produced by a high-frequency oscillator HFO. The output side of the single-sideband modulator is connected with the input of a phase-locked loop PLL whose output signal is converted into the original frequency band of the Doppler signal by means of a demodulator DM. The output of the demodulator is fed to the input of the threshold value switch SW via a low-pass filter TPF defining the output of the discriminator DT.
In Reissued U.S. Pat. No. Re.31,851 dated Mar. 19, 1985, of U.S. Pat. No. 4,359,734 to Bachman, there is illustrated a signal processing system for processing an electrical velocity information signal in a velocity detecting system which includes a detector 10 for developing an electrical information signal having a characteristic that varies systematically with the object's velocity. A converter 30 is used to develop an oscillatory signal having a frequency which varies in accordance with the velocity information signal characteristic. A tracking filter having a phased-lock loop 41 is provided with a center frequency that varies with the oscillatory frequency for filtering all frequencies other than the center frequency. A signal generator is responsive to the frequency of the signal passed by the tracking filter for generating an output signal having a frequency which is a preset ratio relative to the center frequency. The signal generator is comprised of a voltage-controlled oscillator 50, a comparator 60, a charging/discharging circuit 70, a switching circuit 80, and a local oscillator 90. The output frequency from the signal generator has a frequency which represents the velocity of the object being detected.
In U.S. Pat. No. 4,276,548 to Lutz issued on Jun. 30, 1981, there is illustrated a meter for measuring the relative velocity of an object which is comprised of a microwave diplexer 12, a phased-locked loop 14, a lock detector and timer 16, a timer 18, a counter 20, a display 22, and an electronic switch 24. The diplexer 12 illuminates the object with a beam of microwave energy and develops from energy reflected from the object a difference signal having a frequency which is proportional to the relative velocity of the object. The phase-locked loop 14 synchronizes the frequency of an internal oscillator with that of the difference signal and generates a lock signal when synchronization is achieved. The lock detector and timer 16 together with the timer 18 is used to develop a reset signal from the lock signal a predetermined period after synchronization is achieved and a latch signal a predetermined period thereafter. The counter 20 counts the cycles of the internal oscillator which are developed after the occurrence of the reset signal until the occurrence of the latch signal so as to form a sum signal indicative of the relative velocity of the object. The display 22 is provided for displaying the sum signal.
The other patents listed above but not specifically discussed are believed to be of only general interest and to show the state of the art in phase-locked loop systems and their particular application in the field of radar equipment.
However, none of the prior art discussed above teach a digital phase lock detector system for determining the period of an incoming signal like that of the present invention which includes a micro-controller based processor unit operated under a stored program to perform period determination of the incoming signal. The processor unit determines the period of the incoming signal by comparing the difference between successive periods for the purposes of developing a lock condition. The present invention facilitates measuring rapidly and accurately the relative speed or velocity of the target according to the Doppler-radar principle. It represents significant improvements over the aforementioned patents discussed, which are hereby incorporated by reference.